Relation of the Neuroendocrine System to Reproductive Decline in Men

Abstract

In contrast to the sharp landmark provided by the menopause in aging women, reproductive senescence in men appears to be a gradual process. In fact, there is still some question as to whether a true male climacteric occurs at all in normal men. Certainly, some human males retain full reproductive function into extreme old age. For example, Seymour, et al. (1935) documented successful paternity in a 94-year-old man. Nonetheless, considerable published data support the concept that men do show reduced reproductive capacity with age. This phenomenon may seem of little interest in the human male, since only rarely are new offspring desired by men past the age of 50. The subject of sexual function in older men, however, arouses lively interest because, in humans, sexual activity is not necessarily associated with procreation, but rather serves deeply felt personal needs, reinforces the permanence of pair-bonding, and hence is at the root of the stability of human families and, thus, societies. It is important to remember that the same hormones which regulate reproductive function also support and modulate sexual capacity and drive. Alterations in the secretion of these hormones, whether produced by aging or disease, can produce deleterious effects on sex drive and capacity which will be of concern to men of any age. Such alterations could occur as a result of aging changes in the function of the testes themselves. They might result from alterations in the pituitary gland, which secretes the gonadotropic hormones LH and FSH. Finally, aging changes might occur primarily in the hypothalamus, which regulates pituitary gonadotropic function both by secretion of the stimulatory polypeptide, luteinizing hormone-releasing hormone (LHRH), and by release of the neurotransmitter, dopamine, which inhibits pituitary secretion of prolactin. Both LHRH and Dopamine are released by hypothalamic neurosecretory cells into capillaries of the ventral hypothalamus and hence into the pituitary portal circulation.